Method and installation for wastewater process monitoring and control

ABSTRACT

We describe a method of closed-loop control of a waste water treatment plant, the method comprising: obtaining a fluid sample from a fluid of said plant; providing said fluid sample to a sealed chamber such that said fluid sample incompletely fills said sealed chamber leaving a headspace; incubating said fluid sample in said sealed chamber; determining a change in pressure in said headspace during said incubating; and controlling a degree of aeration of said waste water treatment plant responsive to said change in pressure. We also describe a method of measuring one or both of the food content and the biomass content of a fluid of a waste water treatment plant, the method comprising determining a value for one or both of food content and biomass content from a change in pres sure.

FIELD OF THE INVENTION

This invention relates to methods and systems for monitoring andcontrolling waste water treatment plants, in particular sewage plants.This invention also relates to methods and systems for monitoring fluidsin waste water treatment plants, in particular influent and activatedsludge in sewage plants.

BACKGROUND TO THE INVENTION

Waste water treatment accounts for a surprisingly large proportion ofthe total UK energy supply, by some estimates up to 0.5%. The majorityof this energy goes to aeration of the biological floc in a treatmentplant, but this process is relatively poorly understood and often notwell controlled. In the main sight and smell are used by experiencedmanagers to control a waste water treatment plant (when operatingproperly, the smell is not unpleasant), supplemented by occasional testsin the UK typically the BOD5 (biological oxygen demand 5 day test) whichas the name implies, incubates a field sample over five days tocharacterise the sample by its oxygen use. Sometimes probes such as anitrogen, oxygen or ammonia probe are also employed although in practicethese do not work well and often fail.

There is therefore a need for improved techniques for managing wastewater treatment plants, preferably techniques which allow improvedmanagement of shock loads and diurnal/seasonal rhythms.

There is also a need for improved techniques for monitoring fluids inwaste water treatment plants

SUMMARY OF THE INVENTION

Plant Control

According to a first aspect of the invention there is therefore provideda method of closed-loop control of a waste water treatment plant, themethod comprising: obtaining a fluid sample from a fluid of said plant;providing said fluid sample to a sealed chamber such that said fluidsample incompletely fills said sealed chamber leaving a headspace;incubating said fluid sample in said sealed chamber; determining achange in pressure in said headspace during said incubating; andcontrolling a degree of aeration of said waste water treatment plantresponsive to said change in pressure.

The inventors have determined that, surprisingly, the change, moreparticularly drop in pressure in the headspace of a sealed chamber maybe employed to monitor one or both of influent and RAS (returnedactivated sludge) in a waste water treatment plant. The change inpressure is believed to result from combination of use of some gasses,in particular oxygen, in growing bacteria and production of other gassessuch as carbon dioxide, during respiration/bacterial growth.Experimentally an initial pressure drop is observed over a period up toone to a few hours followed by a flattening of the curve and subsequentrise in pressure. The initial drop in pressure has been observedexperimentally to correlate with the food available to the bacteria inthe influent, and with the biomass in the RAS. It has further beenestablished that one or both of these measurements may be employed inclosed-loop control of a waste water treatment plant, with acorresponding loop time, typically of less than 8, 4, 2 or 1 hours.

Controlling the aeration in this manner enables the method (in thecorresponding system) to determine a sufficient level of aerationwithout wasting energy in excess aeration, at the same time ensuringthat the clear output from the waste water treatment plant hassufficiently low BOD for this to be safely discharged into a watercourse. The control may be responsive to, for example, one or more of apressure drop, a rate of pressure drop, and an integrated pressure drop(area under a pressure-time curve).

In some particularly preferred implementations two fluid samples areobtained, one for use in determining a parameter representing a level offood in the influent, another for determining a quantity of livingbiological material (biomass) in the plant. The former sample may beobtained, for example, from the influent to the plant; the latter fromthe RAS (returned activate sludge) corresponding parameters may beobtained from respective changes in headspace pressure when incubatingthese two fluid samples and, without wishing to be bound by theory, itis believed that these parameters represent, respectively levels of foodand biomass in the plant. The degree of aeration may then be controlledresponsive to a combination of these parameters, for example a ratio offood to biomass (although in principle some other combination may beemployed, for example subtracting one parameter from the other).

The particular degree of aeration/control may be determined on aplant-by-plant basis:

typically plants have their own individual characteristics and needs andthe control over the aeration equipment may be adapted accordingly. Inprinciple a plant may be categorised into one of a plurality ofdifferent sizes/profiles of plant and a starting point for a controlprocedure determined accordingly.

In some preferred implementations of the method the fluid sample may beaerated (gassed) prior to incubation to aim to improve the uniformity ofthe initial conditions. Similarly temperature control is preferablyapplied for example either to ensure that all samples are incubated atsubstantially the same temperature, or to incubate a sample atsubstantially the operating temperature of the plant.

In practice it has been found that there is significant noise/variationin the pressure data during an initial period of 5-30 minutes which cangive rise to false/confusing data. Thus in some preferredimplementations data is disregarded during this initial period ofincubation. Likewise changes in available food/gas during the incubationcan affect the pressure drop/drop rate after a period of time of one toa few hours. Therefore in some preferred implementations data obtainedduring an initial period of incubation, and data after 1, 2, 4 or 8hours, are disregarded.

Experimental work by the inventors indicated the difficulty in obtainingreliable results. In situ it was observed that frequently lorry loads ofvarious different types of waste fluid would be delivered to a plant andit was hypothesised that this could result in substantial changes to thegrowing conditions for the bacteria and potential toxicity of the fluid.Experimentally dilution, preferably substantial dilution of the fluidsample helped to produce reliable results, it is surmised by effectivelydiluting out the toxicity. Thus in some preferred embodiments the methodincludes diluting the fluid sample by at least 90%, 95%, 98%, 99% ormore dilution (10% or less original sample remaining) prior toincubating the sample.

Experimental work has indicated that there is a relatively reliablecorrelation between rate of pressure drop, for example pressure drop perhour, and influent food. Thus embodiments of the method may comprisecontrolling the degree of aeration responsive to a determined rate ofchange (drop) of the pressure in the headspace of the sealed chamber.

Experimentally it has been determined that varying the sample toheadspace ratio significantly affects the observed change in pressureand can be used as a mechanism to adjust the sensitivity of themeasurement, in effect the loop gain of the control loop. Again this isa parameter which may be varied from plant to plant. In a similar waythe degree of dilution may also be adjusted to provide control ofsensitivity/loop gain.

In a corresponding aspect the invention provides a control system forclosed-loop control of a waste water treatment plant, the systemcomprising: a culture vessel comprising a sealable chamber for culturinga fluid sample and a pressure measurement transducer for measuring apressure in a headspace of said sealable chamber; and a data processingsystem to: input pressure data from said pressure measurementtransducer; determine at least one parameter relating to said plant fromsaid pressure data; and output data, for controlling a degree ofaeration of said plant, dependent on said at least one parameter; inparticular to determine a degree of aeration for said plant from said atleast one parameter; and output aeration control data, for controlling adegree of aeration of said plant, dependent on a said determined degreeof aeration.

In embodiments the data processing system may be implemented inhardware, or in software, or using a combination of the two. Thus, forexample, the data processing system may comprise a microprocessorcoupled to working memory and to program memory storing processorcontrol code for a procedure to implement the above describedsystem/method. Optionally in embodiments two culture vessels may beprovided, one as a control.

The aeration control data may be output either directly to a controlsystem for the treatment plant or indirectly, for example on a screen orprintout to a user for manual adjustment/control of the aeration system.The aeration control data may indicate a degree of aeration or maysimply comprise a binary or/less indication.

The skilled person will appreciate that the previously describedfeatures of the control method may be implemented in the control system.Thus the embodiments of the control system comprise means forimplementing previously described aspects and embodiments of the method.

Plant Monitoring

According to a first aspect of the invention there is therefore provideda method of measuring one or both of the food content and the biomasscontent of a fluid of a waste water treatment plant, the methodcomprising: obtaining a fluid sample from a fluid of said plant;providing said fluid sample to a sealed chamber such that said fluidsample incompletely fills said sealed chamber leaving a headspace;incubating said fluid sample in said sealed chamber; determining achange in pressure in said headspace during said incubating; anddetermining a value for one or both of said food content and saidbiomass content from said change in pressure.

The inventors have, through experiment, determined that a value forfood/biomass content in fluid from a waste water treatment plant may bedetermined from a change, more particularly a drop, in pressure when thefluid is incubated. This is surprising as the growth/metabolism ofbacteria, protozoa and the like both uses gas (oxygen) and produces gas(CO₂). Without wishing to be bound by theory the overall drop inpressure is believed to relate to the overall production of a largenumber of bacteria in part from the gas in the headspace—without thisone might expect that the gas use and production would approximatelybalance. In some preferred embodiments of the method the fluid sample(s)are aerated to provide a common based line gas level when starting theprocedure, to avoid effects which can otherwise be seen due torestriction in bacterial growth due to gas depletion.

In embodiments of the method the fluid sample comprises a sample ofinfluent to the plant (inflow after the majority of the solids have beenremoved) and the bacterial food content, for example a combination ofoxygen and/or nitrogen and/or phosphorous, of this influent isdetermined. In some preferred embodiments of such a food measuringprocedure the fluid sample is diluted (with water), preferably to a highdegree, prior to incubation, for example at least 90%, 95%, 98%, 99% ormore dilution (that is leaving 10% or less of the original sample). Thisis because toxic materials in the influent can otherwise affect thebacterial growth process and dilution reduces the effective toxicity.

Experimental work has indicated that there is a relatively reliable,close to straight line correlation between the rate of pressure drop,for example pressure drop per hour and the influent food level. Thusembodiments of the method may comprise determining this rate of drop inpressure and then matching this to an influent food level, for examplebased upon a determined straight line (linear) relationship and/or acalibration curve. In preferred embodiments of the method thetemperature is maintained at a substantially constant value, for examplethe temperature of the influent or a fixed or calibration temperaturesuch as 20° C. Alternatively, however, a food level in a fluid from theplant may be determined dependent on one or more of a pressure drop, arate of pressure drop, and an integrated pressure drop (area under apressure-time curve).

In some embodiments of the method the level of food in the influent isdetermined by incubating the influent without RAS (returned activatedsludge). In principle a measurement may be made on a fluid samplecomprising a mixture of influent and RAS, although this is lesspreferable because RAS by itself metabolises and affects (reduces) theheadspace gas pressure—the bacteria grow, die and feed off one another.Nonetheless, in embodiments it can be helpful to add biomass (bacteria)to an influent sample, in particular a very dilute influent sample. Inthis case, preferably a determined quantity (mass) of biomass is added.This may either be some level of biomass weight determined by a protocolof the method, or returned activated sludge, for example from the outputof the plant, may be added to the dilute influent. In the former casethe RAS may be dried, for example gently in a microwave, and adetermined dry weight of biomass added. In the latter case the amount ofbiomass in the RAS may be measured (according to an embodiment of theinvention) and then optionally an adjustment made to the change inpressure to account for any variations in the amount of biomassprovided. In preferred embodiments an excess of the bacteria isprovided, for example by adding substantially more bacteria than wouldin principle be needed to metabolise the food. This is advantageousbecause with a large quantity of bacteria, a relatively significantquantity of gas is consumed/produced even with a low level of food(dilute influent).

Embodiments of the method may additionally or alternatively be employedto determine a level of biomass in RAS from the plant, measuring thebiomass content of the RAS fluid.

Either or both of the food and biomass measurements may be usefullyemployed in controlling a waste water treatment plant, in particular tocontrol a degree of aeration of the plant. In this way embodiments ofthe method may be employed to reduce an overall energy consumption ofthe plant by controlling the level of aeration so that it is notsubstantially greater than that required by the quantity of food and/orbiomass present.

In a related aspect the invention provides a system for measuring one orboth of the food content and the biomass content of a fluid of a wastewater treatment plant, the system comprising: a culture vesselcomprising a sealable chamber for culturing a fluid sample and apressure measurement transducer for measuring a pressure in a headspaceof said sealable chamber; and a data processing system to: inputpressure data from said pressure measurement transducer; and determine avalue for one or both of said food content and said biomass content froma change in pressure measured by said pressure transducer.

In embodiments the data processing system may be implemented inhardware, in software, or using a combination of the two. Thus, forexample, the data processing system may comprise a microprocessorcoupled to working memory and to program memory storing processorcontrol code for a procedure to implement the above describedsystem/method. Optionally, in embodiments two culture vessels may beprovided, one as a control.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows a high level schematic diagram of a waste water treatmentplant;

FIGS. 2 a and 2 b show a culture vessel for use in embodiments of theinvention under, respectively normal atmospheric pressure and reducedpressure;

FIG. 3 shows the variation of pressure with time when incubatinginfluent over a period of hours;

FIG. 4 shows a variation of pressure with time for different ratios ofsample to headspace volume;

FIGS. 5 a and 5 b show, respectively, variation of pressure with timefor different influent dilution levels, and pressure drop per houragainst food level (larger food amounts towards the origin of theX-axis);

FIG. 6 a to 6 c show graphs of pressure against time for varying amountsof influent in combination with a constant amount of biomass (RAS) for,respectively, approximately 10 hours, approximately 100 minutes, andapproximately 20 minutes; and

FIG. 7 shows a schematic block diagram of a control system forclosed-loop control of a waste water treatment plant according to anembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, at a high level, a schematic diagram of the operation of awaste water treatment plant 10. Thus the plant accepts influent 12,fluid from which the solids have been substantially removed, containinga high level of ‘food’ for bacteria, protozoans and the like (‘biomass’)and having a high biochemical oxygen demand (BOD). The output from theplant has two components, a clear component 14 which may be provided toa water course and a biological component 16 comprising livingbiological material referred to as returned activated sludge (RAS),typically at around 60% concentration. The RAS is provided back to theinput side of the plant to help maintain the eco system.

We have previously described a system for monitoring themetabolism/growth of microorganisms, the system comprising a sealedchamber with a flexible diaphragm to provide sensitive pressuremeasurements of gas pressure in the headspace above a culture liquid.For details reference may be made, for example, to US2005/0170497(incorporated by reference).

The inventors have carried out significant experimental work on thesuitability of such a system for application to fluids of a waste watertreatment plant.

FIGS. 2 a and 2 b show, schematically, an embodiment of a similar device100 under, respectively, normal atmospheric pressure and negativepressure (in operation either negative pressure or positive pressure maybe produced). Thus a culture 102 of biological material undergoesmetabolism and growth during which it exchanges gases with the aqueousliquid (water) carrying cells depending upon various factors gas may beused and/or produced, for example the cells may produce carbon dioxideduring respiration. A gaseous headspace 104 of the sealed culturechamber 106 thus experiences changes in pressure due to exchange of gaswith the culture medium, and these are monitored by a diaphragm 108 andconverted to an electronic pressure signal 110 which may, for example,be digitised and processed electronically by hardware, software or acombination of the two. Preferably the system also includes an agitator112 and temperature control (not shown), as well as a sealableinlet/outlet port 114.

Experiments were performed to determine what parameters can be measuredby apparatus of the type illustrated in FIG. 2, in activated sludge andother waste water treatment plant fluids, in order to provide a systemthat reduces the need to aerate activated sludge thus reducingelectricity cost in plant operation: Measuring and managing food tobiomass ratio is an important factor in improving efficiency andlowering energy bills.

Initial experiments determined that the general shape of a pressure-timecurve for influent to a plant is as illustrated in FIG. 3. Thus there isan initial period during which the pressure can vary and results appearunreliable. This typically lasts up to around 10 minutes. The pressurethen begins to fall, flattening out in a trough region 300 after aroundan hour. Over a further period of several hours the pressure thengradually starts to rise once more (the graph of FIG. 3 is not toscale). The initial rate of pressure drop appears to be related to theconcentration of food in the influent, a faster drop being observed withmore food present. Here ‘food’ is used to describe material in all formswhich facilitate the growth of bacteria (including, for example, more orless complex carbon sources, sources of oxygen, nitrogen, phosphorous,and ammonia, and also including, potentially, other bacteria). It issurmised that the pressure drop relates to the conversion of gas intoliving biomass since although oxygen is used during bacterial growth,carbon dioxide is produced. It is further surmised that the troughregion occurs when the oxygen has been depleted, the subsequent smallerpressure rise relating to anaerobic respiration producing carbondioxide. However the inventor does not wish to be bound by theory.

A graph broadly of the shape illustrated in FIG. 3 may be obtained froma sample of fluid from a plant comprising a mixture of influent and RAS,but in practice it can be helpful to separately monitor the influentinput and RAS return paths to facilitate control of a plant based upon afood:biomass ratio.

Sample to Headspace Ratio

An experiment was performed to investigate the effect of the sample toheadspace ratio in the sealed culture vessel. This showed that theliquid phase (sample) to gaseous phase (measured head space) volumeratio can be used to adjust the sensitivity of the test system.

One experimental protocol was as follows:

-   -   1. Fresh, settled (solids removed) influent was stored overnight        at 4-8 Deg C. without aeration. A (normal) small amount of        floating solids remained but very minor.    -   2. Fresh RAS (return activated sludge) was stored overnight at        4-8 Deg C. with aeration.    -   3. Influent was equilibrated to 20 deg C.    -   4. RAS was equilibrated to 20 deg C., washed 3 times in clean        water and mixed 1:1 with Influent.    -   5. RAS/Influent mixture was added to culture vessels at varying        volumes and mixed for 5 minutes open to the air.    -   6. Vessel sealed and logging started in bench rig.

FIG. 4 shows the variation of pressure with time with different samplevolumes: Varying sample volume to headspace ratio gave significantlydifferent pressure drop results, and the variation was reasonablyconsistent. A ratio of ˜1:1 was found to be useful for the particulardevelopment rig employed, with a working volume of ˜100 ml—but theskilled person will appreciate that this is particular to the rigemployed. More importantly the experiments showed that the liquid phaseto gaseous phase volume ratio is one easily modified parameter that canbe adjusted to affect the rate of pressure change. This shows that testprotocol may be modified to account for different test conditions andsensitivity requirements (within limits) if desired.

Some preferred applications of the techniques we describe measure foodto biomass ratio. In one approach the amount of food entering the plantwas measured by measuring just influent, without RAS. The hypothesis wasthat the pressure drop per hour would correlate with the amount ofavailable food.

First Measurement of Food Entering the Plant (Influent)

This showed that the food concentration in the influent entering theactivated sludge vessel can broadly speaking be measured by directlyobserving pressure change associated with metabolic rate ofcontaminating organisms plus the inherent chemical oxygen demand. Acorrelation with BOD (biochemical oxygen demand) seems reasonable.

One experimental protocol was as follows:

-   -   1. Fresh, settled (solids removed) influent was stored overnight        at 4-8 Deg C. without aeration. A (normal) small amount of        floating solids remained but very minor.    -   2. Influent was equilibrated to 20 deg C.    -   3. Dilutions of influent were made in water (temp equilibrated,        not gassed) at 0%, 25%, 50%, 75%, and 100% for testing—to        provide a controlled variation in food level    -   4. 30 ml added to culture vessels and mixed for 5 minutes open        to the air.    -   5. 30 ml water added and left for 3 minutes.    -   6. Vessel sealed and logging started

FIG. 5 a shows the variation of pressure with time with varying degreesof dilution, in effect, the amount of food present. FIG. 5 b shows thatthere is an approximate straight line correlation between the rate ofpressure drop (pressure drop per hour) and the available food—in thiscase the amount of influent, but this could equally be the amount offood in an influent sample. (In FIG. 5 b the left hand side of thex-axis corresponds to a high level of food/influent, and vice-versa).

From FIG. 5 it can be seen that there is a variable pressure dropdependent upon the concentration of influent; that the pressure drop canbe correlated to influent concentration in a straight line relationship(allowing for experimental error); and that the measurement works withina target time course of 60 minutes.

This demonstrates that apparatus of the type illustrated in FIG. 2 canbe used for direct measurement of influent concentration using ameasurement of headspace volume pressure. This correlation of FIG. 5indicates that this technology can be employed to test for the level offood entering the activated sludge process. In practice the measurementmay be a measurement of both BOC and COD (chemical oxygen demand)—but ifso this is potentially advantageous for aeration control. Preferably thesample is aerated (pre-gassed) prior to measurement, to avoid variationsdue to different levels of initial oxygen concentration in the influent.

In an alternative approach, rather than measure just the influent, theinfluent is measured in combination with biomass, in particular RAS.This provides a more particular determination of the bacteria's reactionto the particular food source, and in embodiments the RAS may be derivedfrom the plant being monitored/controlled. Because the RAS itself isactive in the sense that it gives rise to a pressure drop, either aconstant biomass may be employed or the amount of biomass added may bemeasured. A measurement of the biomass may either be made by heating asample, for example by microwaving the sample, to determine the dryweight of biomass or by measuring the amount of biomass indirectly byculturing the biomass as described later. In embodiments incubating thefood source in combination with biomass serves to amplify the signalgenerated by the food since even with a small amount of food, having alarge, more particularly excess quantity of biomass will generate/use amore readily measureable quantity of gas, and hence can provide morerapid results. (Here ‘excess’ bacteria is a quantity of bacteria largeenough that the rate of metabolism of the food is not limited by thequantity of biomass).

Thus further experiments were performed to investigate the incubation ofinfluent in combination with biomass.

Second Measurement of Food Entering the Plant (Influent)

The measurement of food entering the plant using RAS Biomass activityaimed to measure food concentration as a function of Biological OxygenDemand. In embodiments this approach provides a BOD5 test proxy. Theexperiments showed that using high biomass concentration and low foodconcentration, one can mimic the long BOD5 test in a shorter time, forexample of order 1 hour. In embodiments the technique measures rate ofmetabolism as a function of the amount of food available. Thus thetechnique is able to provide a device, in embodiments operating undersoftware control, to rapidly measure the Biological Oxygen Demand ofwater samples using the pressure change and/or rate of pressure changeand/or integrated pressure change, in test sample.

One experimental protocol was as follows:

-   -   1. Fresh, settled (solids removed) influent was stored overnight        at 4-8 Deg C. without aeration. Note: a (normal) small amount of        floating solids remained but very minor.    -   2. Influent was equilibrated to 20 deg C.    -   3. RAS was equilibrated to 20 deg C., unmixed but in large        surface area vessel, shaken every 15 minutes.    -   4. Dilutions of influent were made in water (temp equilibrated,        not gassed). 0%, 2.5%, 5.0%, 7.5%, & 10% for testing    -   5. 30 ml RAS added to culture vessels and mixed for 15 minutes        open to the air.    -   6. 30 ml Diluted Influent sample added to culture vessels and        mixed for 3 minutes open to the air.    -   7. Vessel sealed and logging started

FIG. 6 a shows the variation of pressure with time with varying degreesof dilution, in effect, varying the amount of food present, over aperiod of around 10 hours. FIG. 6 b shows, on an expended scale, thefirst 100 minutes (Phase 1—Ph1—in FIG. 6 a), and FIG. 6 c the first 20minutes (Phase 2—Ph2—in FIG. 6 a).

It can be seen that there is a three stage pressure curve for eachdilution, with a clear difference in pressure drop between samples inphase 1. The phase 1 pressure drop correlates to sample concentration,i.e. the amount of food present. This demonstrates that this approachprovides a feasible substitute for a BOD5 test, but on a timescaleshortened by around two orders of magnitude.

The transition time of Phase 1 to Phase 2, which corresponds to a changein slope, varies between samples and the time of the transitioncorrelates with the amount of food (the larger the amount of food, thesharper the initial pressure drop and the earlier the transition to thegentler slope of phase 2). However the Phase 1-2 timing and rate changesmay also be dependent on the level of oxygenation (the sample is almostanaerobic), and thus preferably the sample is oxygenated prior toincubation/measurement. The phase 2 pressure drop rate is consistentbetween samples.

The time of the transition from phase 2 to phase 3 also varies betweensamples, also apparently correlates with the rapidity of initial oxygendepletion (higher food content samples show more rapid oxygendepletion), although it is harder to see from the curves. The time tothe point at which the pressure drop reaches zero (which may relate tothe time to depletion of the available oxygen) apparently correlateswith the amount of oxygen used by a given biomass, dependent upon thefood availability. The results also apparently correlate with those froma BOD5 test. Similarly the area under the pressure-time curve to thispoint may also be used as an indication of the amount of food availableand, in embodiments, as a proxy for a BOD5 test.

Thus a closed vessel pressure measurement, as previously described, canused as a measure of oxygen utilisation by a given body of biomass withtime, consistent with the food availability.

Some experiments on fluid samples from sewage treatment works showedstrange results that might indicate background toxicity in some influentsamples—for example slightly diluted samples could sometimes appear tohave higher metabolic rate than neat samples. Discussions with plantstaff elucidated that background toxic events can be very common assites often accept tankers of high concentration effluent. Thus dilutionof samples can be useful to remove background toxic or inhibitorycomponents/effects which can otherwise interfere with obtaining accurateresults.

Choosing a suitable level of dilution was found to be important inpractice to see differences between fluid samples, sometimes to see anydifferences. Thus an initial step of characterising a plant to determinea correct dilution range to employ can be important, and in general thedegree of dilution will vary from plant to plant.

Similarly a pre-oxygenation step is also helpful to reduce the risk of atest being unduly influenced by an inherent oxygen level in a sample.More generally, a step equilibrate gaseous composition of biologicallyactive samples or to control the level of gas, in particular oxygen, ina sample is helpful. Temperature control is also useful, in part becauseof the varying gas-dissolving ability of water at differenttemperatures.

In a further set of experiments samples of RAS were diluted and thenincubated using protocols along broadly the same lines as thosedescribed above. The bacteria in RAS metabolise on their own and thus,in a similar way to the approach used for influent, it has beenexperimentally determined that the pressure drop correlates with thebiomass in RAS return. Thus this can be used as a measure of the biomasspresent in RAS return of a water treatment plant. This can then be usedin combination with a measure of food in the plant, for example frominfluent as described above, to determine a ratio of values whichapproximate the food to biomass ratio in the plant. The plant aerationmay then be controlled dependent on this, increasing the aeration whenthere is a large quantity of food for the available biomass, and viceversa.

FIG. 6 shows a block diagram of a closed loop based water treatmentcontrol system 200 to implement real time closed loop control of RASwater treatment based upon measurement or pressure changes in a closedvessel/sealed chamber. Thus one or both of food and RAS samples areprovided to a culture vessel, for example of the type shown in FIG. 2,and the overall changes in gas pressure (a combination of oxygen used anCO² produced, among other potential influencing factors) is monitored bya data processor 210, for example a general purpose computer undersoftware control. The data processor output data, for example as aparameter such as a number indicating the amount of food and/or RAS inthe plant and/or some combination of these such as food to biomassratio. This data may either be output on a screen for an operator toemploy in controlling the plant or the data processor 210 may interfacedirectly or indirectly with an aeration control system 220 for the plantto control the aeration such that it is sufficient, but notsignificantly in excess of that required given the amount offood/biomass the plant is coping with. This in turn enables the plant tooperate efficiently but also to react to shock loads and variations infood/biomass levels over time periods of one or more days, weeks, monthsor years.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

1. A method of closed-loop control of a waste water treatment plant, themethod comprising: obtaining a fluid sample from a fluid of said plant;providing said fluid sample to a sealed chamber such that said fluidsample incompletely fills said sealed chamber leaving a headspace;incubating said fluid sample in said sealed chamber; determining achange in pressure in said headspace during said incubating; andcontrolling a degree of aeration of said waste water treatment plantresponsive to said change in pressure.
 2. A method as claimed in claim 1wherein said fluid sample comprises a sample of influent to said plant.3. A method as claimed in claim 1 wherein said fluid sample comprises asample of returned activated sludge (RAS) in said plant.
 4. A method asclaimed in claim 1, comprising: obtaining a first said fluid samplecomprising a sample of influent to said plant; obtaining a second saidfluid sample comprising a sample of returned activated sludge (RAS) insaid plant; determining respective first and second changes in headspacepressure during incubating of said first and second fluid samplesrespectively; determining respective first and second parameters fromsaid first and second changes in headspace pressure; and controllingsaid degree of aeration responsive to a combination of said first andsecond parameters.
 5. A method as claimed in claim 4 wherein saidcombination of parameters defines a ratio of food to biomass for saidplant.
 6. (canceled)
 7. A method as claimed in claim 1 furthercomprising diluting said fluid sample by at least 90% prior to saidincubating.
 8. A method as claimed in claim 1 further comprisingaerating said fluid sample prior to said incubating.
 9. A method asclaimed in claim 1 wherein said determining of said change in pressurecomprises disregarding changes in pressure during an initial period ofsaid incubating.
 10. A method as claimed in claim 1 wherein saiddetermining of said change in pressure comprises determining a rate ofchange of said pressure to determine a food level parameter relating toa level of food in or supplied to said plant, and controlling saiddegree of aeration responsive to said food level parameter.
 11. A methodas claimed in claim 1 comprising adjusting a volume of said headspace toadjust a sensitivity of said closed loop control.
 12. A method asclaimed in claim 1 further comprising diluting said fluid sample priorto said incubating, and adjusting a degree of said dilution to adjust asensitivity of said closed loop control.
 13. A control system forclosed-loop control of a waste water treatment plant, the systemcomprising: a culture vessel comprising a sealable chamber for culturinga fluid sample and a pressure measurement transducer for measuring apressure in a headspace of said sealable chamber; and a data processingsystem to: input pressure data from said pressure measurementtransducer; determine at least one parameter relating to said plant fromsaid pressure data; and output data, for controlling a degree ofaeration of said plant, dependent on said at least one parameter.
 14. Acontrol system as claimed in claim 13 wherein said data processingsystem is further configured to: determine a degree of aeration for saidplant from said at least one parameter; and output aeration controldata, for controlling a degree of aeration of said plant, dependent on asaid determined degree of aeration.
 15. (canceled)
 16. A method ofmeasuring one or both of the food content and the biomass content of afluid of a waste water treatment plant, the method comprising: obtaininga fluid sample from a fluid of said plant; providing said fluid sampleto a sealed chamber such that said fluid sample incompletely fills saidsealed chamber leaving a headspace; incubating said fluid sample in saidsealed chamber; determining a change in pressure in said headspaceduring said incubating; and determining a value for one or both of saidfood content and said biomass content from said change in pressure. 17.A method as claimed in claim 16 wherein said change in pressurecomprises a fall in pressure, wherein said fluid sample comprises asample of influent to said plant, and wherein said measuring comprisesdetermining a value for said food content of said influent. 18-22.(canceled)
 23. A method as claimed in claim 16 wherein said fluid samplecomprises a sample of influent to said plant, and said determining ofsaid value for said food content of said influent comprises determininga rate of drop in said pressure.
 24. (canceled)
 25. A method as claimedin claim 16 further comprising adding bacteria to said fluid samplecomprising adding one or both of a determined biomass of said bacteriaand an excess of said bacteria.
 26. A method as claimed in claim 25wherein said bacteria comprise RAS (returned activated sludge) from saidplant.
 27. A method as claimed in claim 26 wherein said fluid samplecomprises a sample of influent to said plant and the method furthercomprising obtaining an RAS fluid from said plant and determining alevel of RAS in said RAS fluid; and wherein said determining of saidvalue for said food content of said influent includes compensating for asaid pressure change dependent on said level of RAS.
 28. A method asclaimed in claim 16 wherein said fluid sample comprises a sample byreturned activated sludge (RAS) fluid in said plant, and wherein saidmeasuring comprises determining a value for said biomass content of saidRAS fluid. 29-31. (canceled)